Just curious why you want to change the name of the theme? It doesn’t show anywhere on the site, does it?

To have it show up in the backend with a different name, then you need to start by renaming the directory from crystal to vietfix on the server ( at \wp-content\themes\crystal\). I don’t have the Crystal theme so I am not familiar with how it’s put together so there may be more to it than just renaming the directory, but that’s the first place to start. That change would get it to show up in the WP Dashboard Appearance -> Themes backend screen under your preferred name. (I recommend copying your existing Crystal theme into the new theme and making your modifications there, instead of changing the original.) Doing this means you won’t get notified of updates to the Crystal theme, however.